Smithing

Void Salts are used to improve the Nightingale Armor at a Workbench. The Nightingale Armor, Boots, Gloves, and Hood can be improved by 6, 4, 4, and 2 armor points respectively. Improvement values will scale with Smithing skill level.
They are also an ingredient to make the Storm Atronach spell scroll and tome at the Atronach Forge.

Trivia

This is the one of two ingredients which can be classified a little differently than others. It can be called "Fine Cut" Void Salts, which are rare and are only used as quest items. During the Dawnguard quest Chasing Echoes, "Purified" Void Salts can be found along with "Finely Ground" Bone Meal.